The aluminum nitride has an excellent electrically insulating property and a high thermal conductivity, and it has been expected that a materials such as resin, grease, adhesive and coating material filled with a sintered product or a powder thereof can be used as heat radiating materials featuring a high thermal conductivity.
In order to improve the thermal conductivity of the heat radiating materials, it is important to densely fill the matrix such as resin with a filler having a high thermal conductivity. For this purpose, it has been strongly urged to provide an aluminum nitride powder of a high sphericalness having a grain size of from about several microns to several hundreds of microns.
The aluminum nitride powder has, usually, been produced by an alumina reductive nitridation method which nitrides the alumina in the presence of carbon, a direct nitridation method which reacts aluminum directly with nitrogen, and a gas-phase method which reacts alkylaluminum with ammonia and, thereafter, heats them.
Of them, the aluminum nitride particles obtained by the reductive nitridation method and by the gas-phase method have shapes close to a sphere but their grain sizes are still of the order of sub-microns.
According to the direct nitridation method, on the other hand, the aluminum nitride powder is obtained through the pulverization and classification, making it possible to obtain the aluminum nitride powder having grain sizes of from about several microns to several hundreds of microns. However, the obtained particles are irregular particles having low sphericalness. Therefore, the aluminum nitride powder obtained by the above method cannot be highly densely filled in the resin.
As a method of efficiently obtaining an aluminum nitride powder of a high sphericalness, on the other hand, there has been known a method of producing an aluminum nitride powder by firing a mixed powder of an alumina powder, a powder of an alkaline earth metal compound or a rare earth compound and a carbon powder in a non-oxidizing atmosphere containing nitrogen (see patent document 1).
This method is to form the aluminum nitride at a low temperature of not higher than 1500° C. by utilizing the action of the alkaline earth metal compound or the rare earth compound for promoting the nitridation reaction.
The aluminum nitride particles obtained by the above method have a high sphericalness but their grain sizes are about 1 μm at the greatest, and relatively large grain sizes of the order of several microns have not been realized yet. According to the above method, further, it is difficult to control the grain size of the aluminum nitride powder that is obtained. For example, when the alkaline earth metal compound is used, it has been confirmed that the obtained aluminum nitride powder contains coarse particles of sizes which are unnecessarily large. Besides, it is difficult to separate such coarse particles from the highly adhesive aluminum nitride powder of a grain size of about 1 μm.